FMLA obligation

So I’m a nurse and work 5 2-10 pm shifts. My boss casually told me she’d probably only have overnights when I return from leave. Which I will absolutely not do because they don’t have shift differential. What are my legal obligations to return? The only thing they’re doing is paying the employer contribution for health benefits which I won’t pay back...

Could I come back registry or do I have to come back full time? I was planning on looking for a better paying job.